Hydrostatic transmissions have many uses, including the propelling of vehicles, such as mowing machines, and offer a stepless control of the machine's speed. A typical hydrostatic transmission system includes a variable displacement main hydraulic pump connected in a closed hydraulic circuit with a fixed displacement hydraulic motor. The closed hydraulic circuit includes a first conduit connecting the main pump outlet with the motor inlet and a second conduit connecting the motor outlet with a pump inlet. Either of these conduits may be the high pressure line depending upon the direction of pump displacement from neutral. For most applications, the pump is driven by a prime mover, such as an internal combustion engine or an electrical motor, at a certain speed in a certain direction. Changing the displacement of the pump will change its output flow rate, which controls the speed of the motor. Pump outflow can be reversed, thus reversing the direction of the motor. In a vehicle, the motor is typically connected through suitable gearing to the vehicle's wheels or tracks.
Fluid connections between the pump, motor and conduits should generally be leak free. However, as is often the case, the hoses or other conduits connecting the pump and motor can leak causing a loss of hydraulic fluid and a decrease in transmission performance. In response to the leakage problem, prior art hydrostatic transmissions were created which generally prevent leakage of fluid from the pump and motor by containing the leakage and returning it to the closed hydraulic circuit as needed.
Hydrostatic transmissions generate heat as the hydraulic fluid is circulated between the pump and the motor. Friction between moving parts of the pump and/or motor also generates heat. As hydrostatic transmissions become more compact, heat related issues become more of a problem due to the decreased surface area of the hydrostatic transmission available for dissipating heat.